A signal conversion technology is known which accomplishes nonlinear gradation conversion of image signals based on the dependency between the input gradation value and the average noise amplitude and evenly eliminates the noise across all gradation regions.
For instance, a signal converter that accomplishes gradation conversion according to the following equation is disclosed in Japanese Laid-Open Patent Publication No. 63-290028.                     Y        =                  c          ⁢                                    ∫              b              X                        ⁢                                          [                                  1                  /                                      n                    ⁡                                          (                      ξ                      )                                                                      ]                            ⁢                                                           ⁢                              ⅆ                ξ                                                                        [        1        ]            where X is an input gradation value, Y is an output gradation value, ξ is the variable of integration, n(ξ) is an average noise amplitude in the input gradation value ξ, and b and c are independently determined constants.)
By undertaking both the gradation conversion of this Equation [1] and quantification, it is possible to evenly eliminate the noise regardless of the size of the output gradation value Y, and to effectively reduce the volume of information in the image signal.
Furthermore, by restoring the gradation of the output gradation value Y after quantification using the reverse conversion of Equation [1], it is possible to obtain a restored image signal with reduced noise.
However, the inventor of the invention of the present application conceived the idea of a gradation conversion for equalizing the shot noise that depends on the input gradation value X. (This equalization of the shot noise is not described in Japanese Laid-Open Patent Application No. 63-290028 and had not been known at the time of filing this application.)
However, when the gradation conversion for the equalization of the shot noise was tested, noncontinuous gradation level differences (known as “jumps”) occurred in the image signal after gradation restoration, creating the problem that the rich sense of gradation was easily lost.
In particular, because the dark gradation region is amplified considerably during the course of shot noise equalization, the output gradation value Y becomes a discrete value. Moreover, because minute fixed pattern noise and/or dark current in this dark gradation region are amplified considerably during the course of the shot noise equalization, they become more noticeable.